Electrospray ionization (ESI) is a technique for transporting bio-molecules diluted in a liquid into a gaseous phase. This desolvation method is customarily used for mass-spectrometry identification of proteins. For example, protoleolytic enzymes are employed to digest proteins into unique peptide segments. These segments are then separated through reverse-phase High-Pressure-Liquid-Chromatography (HPLC) and sequentially electro-sprayed into a mass spectrometer. By determining the amino acid sequence of specific peptide segments, the mass-spectrometer yields sufficient information to identify the protein with high confidence.
The fundamental physics of the ESI process has been the subject of numerous investigations (for reviews of recent development in this field, see Bruins A.P. , “Mechanistic aspects of electrospray ionization,” Journal of Chromatography A, vol. 794, pp. 345-347, 1998; and Cech et al., “Practical implications of some recent studies in electrospray ionization fundamentals,” Mass Spectrometry Reviews, vol. 20, pp. 362-387, 2001). An electrospray produces a cloud of ions in the gaseous phase. In a nano-ESI mode favored for applications in proteomics, the electrospray is established by pumping an analyte solution at slow flow rates (100-1000 nl/min) through a small bore capillary placed within a high electric field. When the analyte sample leaves the capillary and enters the high electric field in small droplets, the combined electro-hydrodynamic force on the liquid is balanced by its surface tension, effectively creating a “Taylor cone.” (Taylor G. I., “Disintegration of water drops in an electric field,” Proceedings of the Royal Society of London, vol A280, pp. 383-397, 1964).
The Taylor cone may exhibit different modes of behavior depending on the applied far-field electric field (i.e., voltage divided by the tip to counter-electrode spacing). There are four general regimes of operation for a fixed tip to counter-electrode distance and increasing voltage: (a) a pulsating mode, (b) a constant-amplitude oscillation mode, (c) a “cone-jet” mode, and (d) a “multi-jet” mode at the highest biases. Each mode generates a given distribution of droplet sizes, with each droplet carrying charge. The pulsating mode generally produces droplets of a large distribution in size and charge, which cause fluctuation in total ion current and yield a high degree of non-specific “chemical noise” to the mass spectrum. The pulsating mode also exhibits a pulsing behavior that creates poor reproducibility in signal measurement. In contrast, the constant-amplitude oscillation, cone-jet, and multi-jet modes produce smaller droplets having a higher charge-to-mass ratio and a narrow distribution in both diameter and charge state. The multi-jet mode, however, is undesirable because at such high fields there is a potential for arcing between the tip and counter-electrode. Attempts have been made to optimize the droplet size distribution and ion signal intensities by maintaining the electrospray in the cone-jet mode (note that the stable oscillation mode is sometimes lumped with the cone-jet mode). One approach is to visualize the electrospray nozzle through a microscope or video camera. An operator can then manually adjust parameters such the voltage or the distance between the tip of the capillary and the counter-electrode (i.e., tip to counter-electrode voltage or distance) until a satisfactory spray pattern is achieved. The method, however, requires constant operator attention and adjustment, and does not respond to varying conditions unless the operator observes and reacts to such changing conditions. Recently, PCT publication WO 02/095362 A2 describes an automatic feedback control system for an electrospray nozzle. The automatic feedback control system uses an optical system to monitor the geometry of the Taylor cone and control the spray pattern by adjusting tip to counter-electrode voltage or distance until a desired spray morphology is achieved. This feedback control system, however, requires large, expensive, and delicate optical instruments for image capture and analysis.
Another approach is to monitor the ion current generated by the electrospray process and adjust parameters until an ion current of satisfactory magnitude or stability is obtained. The disadvantage with this approach is that ion current is dependent on the chemical nature of the sample liquid. A change in the chemical composition of the sample liquid will change the ion current. Accordingly, the system must be re-tuned when the chemical composition of the sample liquid changes.
Therefore, a need still exists for an electrospray control system that can effectively control the spray under changing sample conditions to maintain the ionization efficiency.